Perovskite Solar Cells Shine A Little Brighter
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Back in 2015, I interviewed the Chief Technology Officer Dr. Christopher Case of Oxford PV during the InterSolar North America in San Francisco, CA. Dr. Case introduced me to a potentially game-changing solar technology, the perovskite solar cell technology, that holds much promise for lowering the cost and boosting the performance of solar power by increasing photovoltaic efficiency of any solar photovoltaic thin film material to 30+% in a perovskite tandem layer (which is more than the maximum efficiency achieved in traditional mono-and poly-crystalline silicon cells). In laboratories, perovskite cells are manufactured by spin-coating, spraying, or “painting” them onto a substrate (material that provides the surface for the chemicals to crystalize on). Perovskites are only about half a micron thick while silicon layer is roughly 200 microns.
The main hurdle for perovskite is durability. Perovskites are very sensitive to oxygen, moisture, and heat, therefore, requiring heavy encapsulation to protect the cell, leading to increased cost and weight of solar cell. Oxford PV’s tandem cell conversion efficiency is 29.52%. More research and development and data are needed for testing its efficiency, stability, as well as increasing lifespan and replacing toxic materials with safer ones. Company such as Saule Technologies has some very interesting perovskite products in the works: 1. they have a perovskite photovoltaic glass ( a semi-transparent perovskite solar cell printed onto flexible foils and overlayed with layers of glass), making it a window that generates electricity. 2. Saule is also producing energy-harvesting sun-blinds that can block intense summer sunlight, and allowing sunlight to enter the building in mornings and evenings to provide natural light and passive heating. These blinds can be adjusted manually or automatically. 3. In May of 2021, Saule launched the world’s first industrial production line of perovskite solar panels in Poland. Jinko Solar is also working on rolling out perovskite technology.
The perovskite solar cell could be the future of energy, in the video published on Sep. 14, 2021, “Perovskite Solar Cells Could Be the Future of Energy“, below:
Perovskite mineral was discovered over 150 years ago, but it’s only recently that scientists have been able to synthesize the properties of the material in laboratories using commonly available chemicals. And what they’re finding is that it can give a big boost to the performance of existing solar cell technology. This week we take at look at how it works, in the video published on Aug. 9, 2020, “Perovskite Solar Cells: Game changer?“, below:
In the video published on March 26, 2021, “Perovskite solar out-benches rivals-2021| perovskite solar cells shines a little brighter“, below:
Perovskite, a calcium titanium oxide mineral discovered in the Ural Mountains of Russia in 1839. This new old material is generating quite an explosive buzz because scientists have found, in recent years, that it is a great material to be used in solar absorption applications. It can be made simply and inexpensively by using common wet chemistry lab methods and low cost equipment instead of the expensive deposition equipment common in the semiconductor industry. To take a look at how this process is made cheap and accessible, I’m sharing the video below:
These solar (photovoltaic) cells are made in tandem (layer by layer) fashion on a specially coated glass support. In the video above:
- the glass is coated with a dense layer of titanium dioxide, by robotic arm, to prevent electrical charge generated by sunlight from leaking out of the cell.
- a less dense porous oxide layer covers the dense oxide layer (usually titanium dioxide, other oxides may also be used).
- a simple high speed spin coater deposits this layer from solution and spreads this coating evenly across the device.
- heating this glass/device in an oven conditions it for solar cell use.
- prepare the Perovskite material (which absorbs in the broad range of solar spectrum) by combining 2 precursor materials: PbI2 (lead iodide) & CH6IN (methylammonium iodide)
- drip the liquid phase mixture (from 5.) onto the oxide coated device (from 4.)
- spin the resulting device in 6 to assure even coating
- applying halide solution
- heating the device resulting from 8 on a hot plate–>spontaneously crystallizes precursors in freshly deposited liquid
- color changes also result from crystallization process resulting from 9.
Such tandem product has the advantage of being able to be introduced into existing infrastructure of current silicon module manufacturing process, boosting its efficiency. With added few steps toward the end of the production line, the coating (equivalent to second solar cell) takes advantage of the blue portion of the solar spectrum and may improve the solar cell efficiency by 20-25% above the underlying silicon. The fact that Perovskite-based solar cell technology is of earth abundant material also insures its availability and low cost. Its high absorption in solar spectrum enables it to have comparable characteristics to that of gallium arsenide. Its ability to change its sensitivity to different band gaps in solar spectrum allows it to make different architectures in tandem solar cells. It can truly be considered as the Custom Solar Absorber! In short term, Perovskite-based solar cell may boost the efficiency level of existing technology. In the long term, it may be a stand-alone technology with closer efficiency level to that of gallium arsenide but at a much lower cost. It may potentially be sprayed, ink-jet printed, dip-coated, etc. It is no wonder that Dr. Case commented, “the perovskite in solar application is the fastest increasing photovoltaic efficiency of any solar photovoltaic thin film material ever! In just a few years, it went from a lab efficiency of about 6% to well over 17%…the material is a very good solar absorber….bringing the material to 25% efficiency in a monolithic layer and 30+% in a perovskite tandem layer….potentially the future replacement for silicon.”
The perovskite thin-film solar cells, is currently being developed by Oxford PV (a spin-out from the University of Oxford in 2009-2010 to commercialize this technology, which has exclusively licensed the intellectual property developed by Professor Henry Snaith and his team of 20 scientists). Below, Professor Henry Snaith will embellish upon the development of this solar technology, in the video published on Jan. 10, 2014, “Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells“, below :
Henry J. Snaith is Professor of Physics in the Clarendon Laboratory at the University of Oxford and Fellow of the Royal Society. He has pioneered the field of perovskite solar cells and published hundreds of papers. He is founder and CSO of Oxford PV, which holds the largest perovskite patent portfolio worldwide and focuses on developing and commercializing perovskite PV technology. In this interview, he discusses the present status and future prospects of perovskite PV, in the video published on Nov. 11, 2018, “The Path to Perovskite on Silicon PV | Prof. Henry Snaith“, below:
Oxford PV plans on continuing to optimize this technology’s cell efficiency and accelerate the transfer of the technology into production. Furthermore, it aims to develop the range of substrates to which the cells can be applied. With its promising future, we, the solar enthusiasts and investors alike, should keep our eyes on Oxford PV in the coming years. In the next few years, we anticipate that Dr. Henry Snaith and his team of scientists will continue to tackle challenges in trap densities, doping densities, mobility, mechanisms for free carrier generations, etc., to further improve device performance. You will find that many in the solar industry share the optimism of Professor Henry Snaith and Dr. Christopher Case.
In the video published on June 9, 2021, “Henry Snaith – The advent of Perovskite solar cells“, below:
For those of you interested in more details about Perovskite-based solar cell technology, please refer to the two videos below:
1. Introducing Perovskite Solar Cells to Undergraduates:
2. In the video published on Nov. 17, 2017, “Everything you ever wanted to know about perovskite“, below:
Keep in mind that Shockley-Queisser limit applies to silicon solar cell and not to perovskite solar cell. The Shockley-Queisser limit is calculated by examining the amount of electrical energy that is extracted per photon of incoming sunlight.
For better understanding of Shockley-Queisser limit, please refer to the excerpt from wikipedia, in italics, below:
In a traditional solid-state semiconductor such as silicon, a solar cell is made from two doped crystals, one an n-type semiconductor, which has extra free electrons, and the other a p-type semiconductor, which is lacking free electrons, referred to as “holes.” When initially placed in contact with each other, some of the electrons in the n-type portion will flow into the p-type to “fill in” the missing electrons. Eventually enough will flow across the boundary to equalize the Fermi levels of the two materials. The result is a region at the interface, the p-n junction, where charge carriers are depleted on each side of the interface. In silicon, this transfer of electrons produces a potential barrier of about 0.6 V to 0.7 V.
When the material is placed in the sun, photons from the sunlight can be absorbed in the p-type side of the semiconductor, causing electrons in the valence band to be promoted in energy to the conduction band. This process is known as photoexcitation. As the name implies, electrons in the conduction band are free to move about the semiconductor. When a load is placed across the cell as a whole, these electrons will flow from the p-type side into the n-type side, lose energy while moving through the external circuit, and then go back into the p-type material where they can re-combine with the valence-band holes they left behind. In this way, sunlight creates an electric current.
In physics, the Shockley–Queisser limit (also known as the detailed balance limit, Shockley Queisser Efficiency Limit or SQ Limit, or in physical terms the radiative efficiency limit) is the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination in the solar cell. It was first calculated by William Shockley and Hans-Joachim Queisser at Shockley Semiconductor in 1961, giving a maximum efficiency of 30% at 1.1 eV. This first calculation used the 6000K black-body spectrum as an approximation to the solar spectrum. Subsequent calculations have used measured global solar spectra (AM1.5G) and included a back surface mirror which increases the maximum efficiency to 33.7% for a solar cell with a bandgap of 1.34 eV. The limit is one of the most fundamental to solar energy production with photovoltaic cells, and is considered to be one of the most important contributions in the field.
The limit is that the maximum solar conversion efficiency is around 33.7% for a single p-n junction photovoltaic cell, assuming typical sunlight conditions (unconcentrated, AM 1.5 solar spectrum), and subject to other caveats and assumptions discussed below. This maximum occurs at a band gap of 1.34 eV. That is, of all the power contained in sunlight (about 1000 W/m2) falling on an ideal solar cell, only 33.7% of that could ever be turned into electricity (337 W/m2). The most popular solar cell material, silicon, has a less favorable band gap of 1.1 eV, resulting in a maximum efficiency of about 32%. Modern commercial mono-crystalline solar cells produce about 24% conversion efficiency, the losses due largely to practical concerns like reflection off the front of the cell and light blockage from the thin wires on the cell surface.
The Shockley–Queisser limit only applies to conventional solar cells with a single p-n junction; solar cells with multiple layers can (and do) outperform this limit, and so can solar thermal and certain other solar energy systems. In the extreme limit, for a multi-junction solar cell with an infinite number of layers, the corresponding limit is 68.7% for normal sunlight, or 86.8% using concentrated sunlight. (See Solar cell efficiency.)
Gathered, written, and posted by Windermere Sun-Susan Sun Nunamaker More about the community at www.WindermereSun.com
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